Cobalt-base composition

ABSTRACT

An improved cobalt-base braze alloy composition and method for diffusion brazing are provided for use in repairing superalloy articles, such as gas turbine engines, power generation turbines, refinery equipment, and heat exchangers. The improved cobalt-base braze alloy composition includes nickel; at least one element selected from the group of rhenium, palladium, and platinum; at least one element selected from the group of boron and silicon; and the remaining balance consists of cobalt. This composition may also include aluminum, and the composition may be combined with one or more powdered base metal superalloy compositions to form an improved diffusion braze alloy mixture. In the improved method for repairing superalloy articles, the foregoing mixture is applied to a region of the superalloy article to be repaired. The mixture is then heated to melt the cobalt-base braze alloy, thereby joining the base metal superalloy powder particles together, and joining the entire mixture to the region being repaired. The molten mixture is next subjected to a diffusion braze heat treatment cycle in order to break down undesirable boride and silicide phases and to diffuse the melting point depressants into the mixture. In a preferred embodiment, the long term diffusion heat treatment cycle consists of heating the repaired article to 2000° F., holding that temperature for 2 hours, heating the repaired article to 2100° F., holding that temperature for 22 hours, and cooling the article to 250° F. After cooling, an environmental coating is applied to the final repair composite, and this composite significantly improves the cyclic oxidation resistance of the coating compared to the properties of the superalloy base metal.

FIELD OF THE INVENTION

This invention relates generally to diffusion braze repair of superalloyarticles and more particularly to cobalt-base braze alloy compositionscontaining at least one of the following elements: rhenium, palladium,platinum; and to long term diffusion heat treatment of repairedsuperalloy articles.

BACKGROUND OF THE INVENTION

High temperature operating environments such as those present in gasturbine engines, power generation turbines, refinery equipment, and heatexchangers demand parts composed of a variety of cobalt-, iron-, andnickel-base metals known as superalloys. These superalloys are capableof withstanding extremely high temperatures for extended periods oftime, but the extremely stressful temperature conditions to whichsuperalloy articles are subjected eventually takes its toll upon themetal in a number of ways.

The main types of damage to a superalloy article are cracks from thermalfatigue, wide gap cracks, foreign object impact damage, and dimensionalreduction from mechanical wear. Because the cost of these superalloycomponents is quite high, there is considerable incentive to repairthese types of defects rather than to scrap the part and replace it witha new one. The high cost of these components, as well as the fact thatsuperalloy components, once damaged, tend to fail repeatedly in the sameregion, also makes it critical that any repairs made have mechanical,environmental, and processing properties equivalent to or better thanthe original superalloy base metal.

Traditional methods for repairing damaged superalloy articles involvechoosing or creating an alloyed combination of elements that will meltat a temperature below the melting temperature of the superalloysubstrate. These compositions are known in the industry as braze alloys,and the most useful prior art braze alloys are characterized as eithernickel-base or cobalt-base alloys. Historically, the most popular brazealloys contain a melting point depressant such as silicon or boron; acomplex of some of the same alloying elements used in the superalloyarticle to be repaired such as chromium, aluminum, titanium, tungsten,etc.; and either nickel or cobalt as the base. In fact, one braze alloy,sometimes known as B-28, is simply the combination of an alloyfrequently used to manufacture cast turbine airfoils, named Rene'80,with about 2% boron.

Advances in the braze alloy composition art have broughtmulti-constituent alloy compositions which are mixtures of at least onebraze alloy and at least one base metal alloy, the base metal alloydiffering from the braze alloy in that it melts at a higher temperaturethan the braze alloy and contains no melting point depressants which canweaken the repair site. These multi-constituent compositions result instronger repairs because the low-melting brazing alloy liquefies first,wetting the base metal constituent and joining the entire mixture to thesuperalloy article.

Once a braze alloy or alloy mixture has been chosen, the damagedsuperalloy article is cleaned to remove any environmental coating thatmay be over the base metal and any oxides that may have developed insidethe damaged regions. The braze alloy composition is then applied to theregion to be repaired, and the article subjected to a high temperaturebrazing cycle to melt and join the braze alloy to the superalloyarticle. Upon the completion of this cycle, typical braze alloys willhave formed undesirable large blocky or script-like brittle phasescomposed of chromium, titanium, and the family of refractory elements(e.g., tungsten, tantalum) combined with the melting point depressants.These brittle phases weaken the repair composite and cannot be removedfrom conventional braze alloys.

However, certain braze alloy compositions, known as diffusion brazealloys, are capable of withstanding higher temperatures thanconventional braze alloys. Diffusion braze alloys form the same badphases during brazing as conventional alloys, but diffusion braze alloyscan be subjected to a second, long-term high temperature heat cycleknown as a diffusion cycle. This diffusion cycle allows the brittleborides, carbides, and silicides to break down into fine, discreteblocky phases. The diffusion cycle also diffuses the elemental meltingpoint depressants into the braze alloy matrix. These actions result in astronger repair that is less susceptible to incipient melting when thepart is returned to service.

Unfortunately, the diffusion braze alloys of the prior art have failedto attain the crucial part-like mechanical and environmental propertiesdemanded by the increased stresses to which today's superalloy articlesare subjected. The main reason for this failure is that prior hightemperature braze alloys and alloy powder mixtures tend to use onlythose elements present in the superalloy article being repaired.

This lack of flexibility in the compositions of the prior art has causeda stagnation in the development of truly new braze alloy compositionswhich employ elements and elemental combinations without regard to thecomposition of the superalloy substrate. As well, previousmulti-constituent alloy compositions were so precisely matched to theparticular superalloy to be repaired that it was considered unthinkableto select base metal powders for the mixture based solely on theirmechanical and environmental properties.

A need therefore exists for a flexible diffusion braze alloy systemcapable of accommodating various new elements and base metal powders toincrease the strength, flow characteristics, and oxidation resistance ofthe braze alloy system. A need also exists for a long-term diffusionheat treatment cycle capable of breaking down brittle phases andallowing the elemental melting point depressants to diffuse both intothe superalloy substrate and the base metal matrix. A further needexists for a diffusion braze alloy composition which does not rely uponintentional carbon additions for strength. Additionally, a need existsfor a diffusion braze alloy composition capable of imparting improvedenvironmental resistance to the superalloy substrate and/or anyenvironmental coating which may be applied to the substrate.

Such a new diffusion braze alloy system desirably employs the elementsrhenium, platinum, palladium and/or aluminum in order to improvesignificantly over the hot corrosion and oxidation resistance propertiesprovided by prior art braze alloys. Additionally, such an improved brazealloy composition preferably uses boron and silicon concurrently asmelting point depressants in order to reduce the undesirable mechanicaland environmental properties associated with the use of either boron orsilicon alone. The present invention addresses these needs.

SUMMARY OF THE INVENTION

Briefly describing one aspect of the present invention, there isprovided an improved cobalt-base braze alloy composition and method fordiffusion braze repair of superalloy articles that achieves mechanical,processing, and environmental properties equivalent to and, in manycases, better than those properties possessed by the superalloyarticles. The present cobalt-base braze alloy composition comprisesnickel; at least one element selected from the following group: rhenium,palladium, platinum; boron; silicon; and cobalt. This composition mayalso include aluminum, chromium, titanium, tungsten, molybdenum,niobium, hafnium, tantalum, iron, manganese, yttrium, and/or zirconium,which elements appear in many advanced superalloy base metalcompositions. This cobalt-base braze alloy composition may be combinedwith one or more powdered base metal superalloy compositions to form animproved diffusion braze alloy mixture having enhanced mechanical,environmental, and processing properties compared to prior art brazealloy mixtures. The present invention also provides new cobalt-base basemetal alloy compositions for use in such improved diffusion braze alloymixtures, which base metal alloy compositions do not include meltingpoint depressants but which are otherwise similar to those of the brazealloy compositions.

The present invention employs melting point depressants such as boron,silicon, and aluminum to reduce the melting point of the braze alloy.Although the present braze alloy compositions contain relatively lowamounts of melting point depressants, these depressants nonethelessadversely affect the mechanical and/or environmental properties of arepaired article unless they are subjected to a long-term diffusion heattreatment cycle.

The present invention therefore describes an improved diffusion heattreatment method to break down the undesirable phases formed by themelting point depressant(s) and diffuse the depressant(s) into the basemetal alloy matrix. In this way, this long-term diffusion heat treatmentmethod minimizes the negative properties associated with the use ofconventional melting point depressants.

In the brazing method of the present invention, a damaged region of asuperalloy article is repaired by first cleaning the article by anyconventional means; preparing a braze alloy composition mixtureaccording to the present invention, wherein the mechanical andenvironmental properties of that mixture are chosen to equal andpreferably improve upon those properties of the superalloy article to berepaired; depositing this mixture on the region to be repaired; andplacing the superalloy article in a furnace under an inert gasatmosphere or under a vacuum. Once in such a furnace, the pressure inthe furnace chamber should be reduced to approximately 1×10⁻³ torr or alower pressure and the brazing cycle initiated by heating the repairedregion to a temperature of about 800° F. The 800° F. temperature ismaintained for approximately 15 minutes, whereafter the temperature isincreased to about 1800° F. and that temperature maintained forapproximately 15 minutes. Next, the temperature is again raised to 2225°F. and that temperature maintained for between 15 and 45 minutes.Finally, the furnace is vacuum cooled from about 2225° F. to about 1800°F. This step completes the conventional brazing cycle which causes theformation of undesirable brittle phases. The next steps in the presentmethod constitute the diffusion heat treatment cycle that will breakdown these brittle phases.

Upon completion of the high temperature brazing cycle, the superalloyarticle is subjected to a pressure higher than the pressure used in thebrazing cycle and reheated to a temperature of between 1 and 400° F.below the chosen brazing temperature for the article. This temperatureis maintained for at least 20 hours, whereafter the temperature islowered to about 250° F. At this point, the superalloy article is fullyrepaired and ready for machining.

The superalloy article is then usually coated with a metal or ceramic,diffusion or overlay coating according to any known application method.This coating protects the superalloy base metal from oxidation and hotcorrosion attack, and, if the superalloy article is given a multi-layercoating of which at least one layer is a cobalt-base braze alloyaccording to the present invention, the coating remains resistant toenvironmental attack much longer than a traditional coating.

One objective of the present invention is to provide a cobalt-basediffusion braze alloy composition including one or more of the elementsrhenium, palladium, and platinum which elements enhance the solidsolution strength of the repair composite to such an extent that othersolid solution strengthening elements considered undesirable foroxidizing environments and normally contained within conventional brazealloy compositions may be reduced or removed entirely from the brazealloy composition.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy containing elements such as platinum and palladiumwhich improve the flow characteristics of the braze alloy and impartmechanical, processing, and environmental resistance properties to therepaired region equal to or better than those of the superalloy article.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy composition including one or more of the elementsrhenium, platinum, and palladium which elements enhance and improve theoxidation resistance of the repair composite compared to the oxidationproperties of the superalloy article.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy having no intentional carbon additions, whichremoval of carbon thereby reduces the amount of carbides in the repaircomposite and prevents the agglomeration of carbides at the interface ofthe repair composite and the superalloy article.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy composition including aluminum in order to achievegamma-prime strengthening of the repair composite, in order to enhancethe oxidation resistance properties of the repaired region, and toreduce the melting point of the diffusion braze alloy composition.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy composition employing concurrent boron and siliconadditions as melting point depressants in order to minimize thedetrimental properties associated with use of boron and silicon alonewhile still achieving a reduced melting point, desirable flowcharacteristics, and oxidation resistance.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy composition capable of incorporating andefficiently diffusing throughout one or more base metal alloy powders ina braze alloy mixture in order to impart mechanical, processing, andenvironmental resistance properties to the repaired region equal to orbetter than those of the superalloy article.

Another object of the present invention is to provide a cobalt-basediffusion braze alloy composition capable of improving the adhesion ofan environmental coating deposited upon the superalloy article andthereby improving the environmental resistance of the superalloyarticle.

Another object of the present invention is to provide a cobalt-base hightemperature base metal alloy including one or more of the elementsrhenium, palladium, platinum, and aluminum and not including carbon,whereby the base metal alloy enjoys all the benefits such elementaladditions and deletions impart as discussed above.

A further object of the present invention is to provide a method forrepairing superalloy articles which incorporates a long term diffusionheat treatment cycle to break down the brittle phases associated withthe use of melting point depressants such as boron, carbon, and silicon.

These and other objects, advantages and features are accomplishedaccording to the compositions and methods of the following descriptionof the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the preferred embodimentsthereof, and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations, modifications, andfurther applications of the principles of the invention beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The principal objective of the present invention is to achievemechanical, processing, and environmental resistance properties in abraze alloy repair composite that equal, if not exceed, the levels ofthese properties enjoyed by the superalloy substrate, or base metal.Prior art braze alloy compositions have failed to achieve this objectivefor several reasons. First, prior art braze alloy systems are unable toreduce the melting point of the brazing alloy without embrittling eitherthe repair composite or the superalloy substrate. Second, skilledartisans have for so long considered it crucial that the elements of thebraze alloy system match those of the superalloy article to be repairedthat it was thought impossible or unworkable to repair a superalloyarticle using any other elements. And lastly, this old way of thinkingabout braze alloy compositions prevented artisans from investigatingwhat base metal alloys could be added to a braze alloy system to improvea repair's solid solution strengthening and oxidation resistanceproperties.

It is clear that braze alloy systems having the foregoing problemscannot effectively repair today's higher temperature and higher strengthsuperalloys which undergo greater mechanical and thermal stresses thanever before, and which cost more to manufacture than ever before.Therefore, the compositions and method of the present inventionintroduce new elements and elemental combinations not previouslyconsidered for use in the field of diffusion braze repair in order toovercome the disadvantages of the prior art and to give the superalloycomponent a longer useful life than was previously consideredattainable.

The cobalt-base diffusion braze alloy composition of the presentinvention has the following general composition range, by weight:

    ______________________________________    Elements      Weight Percent    ______________________________________    Cobalt        Balance    Nickel        0.001-<Co    Chromium       0-40    Aluminum       0-12    Titanium      0-6    Tungsten       0-15    Molybdenum     0-15    Niobium        0-12    Rhenium       0.001-15    Hafnium       0-6    Tantalum       0-15    Platinum      0.001-40    Palladium     0.001-40    Iron          0-3    Manganese     0-1    Carbon          0-2.0    Boron         0.001-6    Silicon       0.001-10    Yttrium       0-2    Zirconium     0-2    ______________________________________

While the foregoing constitutes a general description of the cobalt-basebraze alloy composition of the present invention, the following arespecific examples of preferred compositions according to the presentinvention. These specific examples are provided for purposes ofillustrating the invention, and no limitations on the invention areintended thereby. Before proceeding further, it should also be notedthat when zeroes are used in the composition tables of the presentinvention, those zeroes indicate no intentional addition of the element,not that the element is absent from the composition. It is noteconomically feasible to use 100% pure elemental additions, andtherefore some impurities may be introduced into the composition.

A first preferred embodiment of the cobalt-base braze alloy compositionof the present invention is known as "RCA-C1" and has the followingcomposition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10.5    Chromium     23    Aluminum     1.5    Titanium     1.75    Tungsten     3    Rhenium      1    Tantalum     6    Platinum     0-40    Palladium    0-40    Carbon         0-0.55    Boron        1.5    Silicon      5    ______________________________________

A second preferred embodiment of the cobalt-base braze alloy compositionof the present invention is known as "RCA-C2" and has the followingcomposition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10    Chromium     22.5    Titanium     0.1    Tungsten     7    Rhenium      0.001-15    Tantalum     3.5    Platinum     0-40    Palladium    0-40    Carbon        0-0.6    Boron        1.5    Silicon      5    Zirconium    0.5    ______________________________________

A third preferred embodiment of the cobalt-base braze alloy compositionof the present invention is known as "RCA-C3" and has the followingcomposition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10.5    Chromium     20.5    Aluminum     2.25    Tungsten     1.25    Rhenium      1    Tantalum     7.75    Platinum     0-40    Palladium    0-40    Carbon         0-0.28    Boron        3    ______________________________________

A fourth preferred embodiment of the cobalt-base braze alloy compositionof the present invention is known as "RCA-C6" and has the followingcomposition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       30.75    Chromium     14.75    Aluminum      3.38    Tungsten      1.38    Rhenium      0.5    Tantalum     8.8    Hafnium      0.5    Platinum     0-40    Palladium    3    Carbon        0-0.3    Boron         2.33    Silicon       3.38    ______________________________________

A most preferred embodiment of the cobalt-base braze alloy compositionof the present invention is known as "RCA-C4" and has the followingcomposition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10.5    Chromium     23    Aluminum     1.75    Tungsten     1.25    Rhenium      1    Platinum     0-40    Palladium    0-40    Tantalum     6.5    Carbon         0-0.55    Boron        2.15    Silicon      3.25    ______________________________________

Turning now to discuss the novelty of the foregoing compositions, itwill be obvious to one of ordinary skill that certain preferredembodiments of the instant diffusion braze alloy compositions areformulated with concurrent boron and silicon additions as melting pointdepressants. Prior art braze alloys, in contrast, have traditionallyused boron alone as the melting point depressant for two major reasons:(1) boron diffuses exceptionally well into the base metal matrix of abraze alloy mixture, and (2) this boron diffusion results in a higherremelt temperature of the final repair composite. Boron in a braze alloythus ensures that the repair composite will be able to withstand thesame high temperatures withstood by the superalloy substrate itself.

However, exceptionally high boron concentrations in a braze alloypromote embrittlement of the superalloy base metal and incipientmelting. These deleterious effects reduce the number of repairs that canbe performed upon any one region of a superalloy article and therebyshorten the operating life of the part since superalloy components tendto fail repeatedly in the same area.

Silicon alone is typically used in conventional (non-diffusion) brazingalloy compositions to speed the alloy's rate of flow into a damagedregion. Unfortunately, silicon-only braze alloys do not typically have ahigh degree of diffusivity into the base metal matrix, and they tend toform very stable silicides. These silicides form large, brittle,script-like phases in the microstructure of the repair composite whichcan degrade the mechanical properties of both the repair composite andthe superalloy base metal.

Embodiments of the present invention combine the two elements tominimize the undesirable effects of using either boron or silicon aloneand maximize the beneficial properties imparted by each element. Whenboron and silicon are combined, the amount of boron necessary to reducethe melting temperature of the alloy is decreased, which reduces thehigh concentrations of boron in the superalloy substrate. The instantbraze alloy system thus enjoys the strength and high temperature meltingproperties imparted by boron without having to sacrifice the superalloybase metal in the process.

Similarly, the silicon additions in the present braze alloy compositionsimprove the flow characteristics of the braze alloy without embrittlingthe repair composite with large amounts of script-like silicide phases.This latter benefit is assured when the long-term diffusion heattreatment cycle of the present invention is used to homogenize the brazealloy/base metal mixture and diffuse the elemental boron and siliconinto the base metal matrix. Silicon also has the unexpected benefit ofimproving the performance of any environmental coating placed over therepaired region. This feature helps assure long life of the repairedarea and gives it improved environmental resistance properties over theoriginal superalloy substrate.

It should be understood, however, that the use of either boron orsilicon alone as a melting point depressant is also considered andintended to come within the scope of the present invention. As will bediscussed in greater detail below, the use of palladium, platinum, andespecially rhenium in the preferred compositions of the presentinvention significantly reduce the deleterious brittle phases associatedwith the use of boron alone and thereby help to increase the re-melttemperature of the final repair composite. The present compositions,therefore, achieve unexpected results over traditional boron- orsilicon-only diffusion braze alloys.

Cobalt-base base metal alloy compositions are also intended to comewithin the scope of the present invention. As discussed previously,braze alloy compositions may be used alone to repair part damage, butsignificant benefits in mechanical strength and processing propertiescan be achieved when a part is repaired using a mixture of one or morebraze alloys and one or more base metal components. The main reason forthese improvements over single-component braze alloy systems is that theamount of melting point depressants used can be significantly reduced.To achieve such property improvements, then, the present invention hasdescribed braze alloy compositions which may be combined with any knownsuperalloy base metal to create an improved repair composite. Thefollowing discussion describes new base metal alloy compositions whichcan be combined with the instant and/or any other known braze alloycompositions to also create an improved repair composite.

The base metal alloy compositions described herein possess the samegeneral composition range as the braze alloy compositions of the presentinvention, but obviously do not include boron or silicon. Therefore, theinstant base metal alloy compositions comprise generally, by weight:

    ______________________________________    Elements      Weight Percent    ______________________________________    Cobalt        Balance    Nickel        0.001-<Co    Chromium       0-40    Aluminum       0-12    Titanium      0-6    Tungsten       0-15    Molybdenum     0-15    Niobium        0-12    Rhenium       0.001-15    Hafnium       0-6    Tantalum       0-15    Platinum      0.001-40    Palladium     0.001-40    Iron          0-3    Manganese     0-1    Carbon          0-2.0    Yttrium       0-2    Zirconium     0-2    ______________________________________

While the foregoing constitutes a general description of the cobalt-basebase metal alloy composition of the present invention, the following arespecific examples of preferred compositions according to the presentinvention. These specific examples are provided for purposes ofillustrating the invention, and no limitations on the invention areintended thereby.

A first preferred embodiment of the cobalt-base base metal alloycomposition of the present invention is known as "RCA-B1" and has thefollowing composition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10    Chromium     22.5    Aluminum     2    Tungsten     5    Rhenium      0.5    Tantalum     6    Platinum     0-40    Palladium    0-40    Carbon         0-0.55    Zirconium    0.5    ______________________________________

A most preferred embodiment of the cobalt-base base metal alloycomposition of the present invention is known as "RCA-B2" and has thefollowing composition:

    ______________________________________    Elements     Weight Percent    ______________________________________    Cobalt       Balance    Nickel       10.5    Chromium     22    Aluminum     1.75    Tungsten     4    Tantalum     6.5    Rhenium      0-15    Palladium    0-40    Platinum     0.001-40    Carbon         0-0.55    ______________________________________

As can be seen in all the foregoing diffusion alloy compositions, brazealloys and base metal alloys alike, the instant alloy compositionscontemplate use of one or more elements from the following group:rhenium, palladium, and platinum. The use of rhenium, palladium andplatinum in cobalt-base diffusion alloys represents a significantadvance in the art of diffusion braze repair of superalloy articlesbecause it departs radically from the traditional diffusion braze alloycomposition: a powder of the same superalloy as the damaged componentwith a measure of melting point depressants added to lower the brazingtemperature. These new alloy compositions are formulated to not onlyrepair, but also to improve, the mechanical, processing, andenvironmental properties possessed by the superalloy base metal. It iswell known that failures in superalloy components regularly occur in thesame or an immediately adjacent location. It is therefore extremelyimportant that these areas of fatigue be repaired to be even strongerthan the original superalloy base metal. The compositions of the presentinvention achieve this objective by successfully combining certainelements such as rhenium, platinum group elements, and aluminum, and byremoving carbon from the compositions.

The first of these new preferred elements, rhenium, is preferably addedto the cobalt-base alloy compositions of the present invention in anamount from 0 to 15 weight percent. Rhenium additions give the presentcompositions significantly improved mechanical and environmentalproperties over other, more traditional, solid solution strengtheningelements such as tungsten, molybdenum, or hafnium. The mechanicalproperties associated with rhenium compositions are similar to thoseachievable by using tungsten and molybdenum; however, rhenium hassignificant oxidation resistance properties that the tungsten andmolybdenum-type elements do not have. Therefore, the inclusion ofrhenium in the compositions of the present invention permits a skilledartisan to reduce or completely remove other solid solutionstrengthening elements that are undesirable for use in oxidizingenvironments. Of additional benefit to the preferred compositionembodiments, rhenium does not promote sigma phase formation in therepair composite or the adjacent superalloy base metal.

Another benefit of rhenium-containing compositions according to thepresent invention relates to rhenium's effect on melting pointdepressants in the alloy matrix. Unexpectedly, the addition of rheniumto the present preferred braze alloy compositions works so well to bindup significant amounts of melting point depressants that the elementswhich traditionally form brittle phases (e.g., chromium, tungsten) areleft in solid solution to strengthen the repair composite and improveenvironmental resistance. As well, the instant preferred compositionseliminate the diffusion of excess melting point depressants into theadjacent base metal of the superalloy article. This is true even whensilicon is not used concurrently with boron, and the amount of meltingpoint depressants which can be successfully incorporated in the alloymatrix increases with the length of the long-term heat treatmentdiffusion cycle. The present compositions can therefore use boron aloneto lower the melting temperature of the braze alloy and achieve thebenefit of a higher re-melt temperature for the repair composite withoutexperiencing the weak and destructive brittle phases or the excess borondiffusion experienced with the prior art boron-containing braze alloys.

Platinum, the second preferred new element, may be added to the presentcompositions in a range of from 0 to 40 weight percent. The addition ofplatinum and/or other platinum group elements, such as osmium, rhodium,iridium, and palladium, improves the hot corrosion and oxidationresistance properties of the repair composite. As well, platinum andother platinum group metals added to the present invention insufficiently high concentrations improve the ductility, or plasticity,of the repair composite.

The addition of palladium is contemplated by the present inventionbecause it achieves improvements in the repair composite similar tothose achieved by platinum. For example, palladium enhances theoxidation resistance of the repair site and improves the ductility ofthe repair composite. Palladium also enhances the flow characteristicsof the instant braze alloy compositions, and nickel and palladium are100% soluble when combined in a braze alloy mixture. Further, palladiumadditions have been shown to retard the formation of undesirable boridesand suicides in the alloy matrix.

Another element contemplated by and intended to come within the scope ofthe present invention is aluminum. Conventional high temperature brazealloys such as AMS 4783 do not have aluminum in them. This is becausethe aluminum reduces the flowability of the braze by the rapid formationof aluminum oxide, a material commonly used for the prevention of brazeflow. Additionally, different surface tensions and viscosities occurwhich change the braze flow characteristics when aluminum is used.Because diffusion braze alloys do not have the same flow requirements asconventional braze alloys, diffusion braze alloys allow the use ofaluminum. Nonetheless, aluminum is not normally used in cobalt-basesuperalloy repair because prior art repair systems typically usepowdered cobalt superalloys combined with a braze alloy to repair acobalt superalloy substrate, and cobalt superalloys do not containaluminum.

Cobalt superalloys are typically used in the temperature range at whichthe superalloy base metal is subject to hot corrosion attack and damage.Certain turbine manufacturers have recently begun to push the operatingtemperatures for cobalt superalloys above this temperature range andinto the oxidation mode of base metal attack and damage. It is for thisreason that the present invention includes aluminum in a cobalt-basediffusion braze alloy composition. By including aluminum in the instantcompositions, the final repair composite receives additional protectionfrom preferential oxidation at the repaired areas of the superalloycomponents; the gamma prime phase of the alloy matrix is strengthenedover non-aluminum containing cobalt-base braze alloys; and theintroduction of aluminum helps reduce the melting point of the brazealloy composition. These benefits outweigh any previously encountereddifficulties with braze flow characteristics, and the inclusion ofaluminum represents a significant advance in the diffusion braze alloyart.

It is well known in the art that, other than using solid solutionstrengthening elements, carbides are the primary strengthening mechanismfor cobalt-base alloys. Because the compositions of the presentinvention include such effective solid solution strengthening elementsas rhenium, and because the present compositions contemplate the use ofsilicides and/or borides to strengthen the alloy matrix as effectivelyas carbides, carbon may effectively be removed from the presentcompositions without suffering any loss in mechanical properties.

It is particularly beneficial to remove carbon from diffusion alloycompositions because carbon prefers to agglomerate and precipitate outof the alloy matrix at lower temperatures. Carbides therefore exhibitpoor ductility and have poor oxidation resistance. Carbide particles ina cobalt-base alloy system also tend to go into solution in the alloymatrix and disappear at high temperatures. However, as soon as thesuperalloy cools, the carbides precipitate out of the matrix and form acarbide line at the interface of the repair composite and the superalloysubstrate. This carbide line allows the repair composite to break awayfrom the superalloy substrate in a zipper-like fashion. The merepossibility of such a significant repair failure makes removing carbonfrom the present invention a significant improvement in the art.

Of importance, the most preferred embodiments of the presentcompositions are prealloyed powders. The prealloying is accomplishedusing well-known methods according to the following procedure: the basicelements are first mixed in the required weight percentages in acontainer; this mixture is then melted at high temperature; and themolten mixture is atomized by spraying the metal through a high pressurenozzle and cooling it with argon gas. This technique solidifies the oncediscrete elements into uniform powder particles. Skilled artisans willrecognize that the properties of a prealloyed mixture are significantlydifferent from those of a simple mixture of elements, and theimprovements achieved by the present invention rely in part upon thefact that these compositions are prealloyed.

The present alloy compositions contemplate the inclusion of a number ofother elements typically used in advanced superalloy compositions,including solid solution strengtheners such as cobalt, molybdenum, andtungsten; gamma-prime formers such as nickel, hafnium, niobium,titanium, and tantalum; sacrificial oxide formers such as chromium;carbide formers such as zirconium; elements to improve ductility such asmanganese; and other elements such as iron. Because these elements arecommonly used in superalloy base metals and braze alloys and because theproperties they impart to those systems are well known in the art, thoseof ordinary skill will understand which elements to choose to customizethe instant compositions to their specifications.

Having now described the preferred composition formulations of thepresent invention, it is necessary to discuss the preferred mixtures foruse in repairing a damaged superalloy component. It is known in the artof superalloy repair that combining in a braze alloy mixture a hightemperature melting composition and one or more compositions which meltat a lower temperature will improve the strength of the repair compositewhile still providing adequate flow characteristics to facilitateplacement and insertion of the braze alloy system into the damagedregion. However, the high temperature component used in prior mixtureswas nothing more than a powder of the same superalloy as the articlebeing repaired.

The present invention, in contrast, describes a diffusion braze alloysystem which employs base metal powders chosen without regard to thecomposition of the superalloy substrate. Instead, the present inventionchooses which base metal powders to incorporate based on the propertiesthose base metals will impart to the braze alloy system or, the repaircomposite. In certain preferred embodiments of the present invention,the use of multiple base metal components, whether iron-, cobalt-, ornickel-base, enhances the mechanical, environmental, and processingproperties of the instant braze alloy system.

As an example, one base metal powder may be chosen for its strength andanother base metal powder chosen for its improved braze flowcharacteristics. One preferred embodiment of the mixture of the presentinvention uses a base metal alloy powder known in the industry asMar-M509. Mar-M509 is known to provide a very strong repair composite,but it is not preferred for use in diffusion braze repair because itslows the flow of molten braze mixture during the high temperature brazecycle. This slow flow characteristic is especially undesirable when thedamage to the superalloy article is in the form of a crack or a widegap. It is therefore desirable when repairing cracks and gaps to includea second base metal powder known in the industry as X40. When usedalone, X40 makes for a relatively weak repair composite, but whencombined with Mar-M509, it improves the flow characteristics of thebraze alloy system and permits cracks and gaps to be filled with astronger repair composite. Certain other preferred embodiments of thepresent invention choose the high temperature base metal alloycompositions of the present invention in order to impart the improvedproperties associated with those base metal powders to the braze alloymixture.

Although the following may generally be known in the industry, it isinstructive for practicing the present invention that in the embodimentsof the present braze alloy mixtures preferred for repairing cracks, thebraze alloy composition or compositions comprise no more than 50% byweight of the total braze alloy mixture. Wide cracks and gaps may berepaired with the present mixtures if the percentage by weight of thebraze alloy composition or compositions is kept to about 40%. Similarly,dimensional repairs, or build-ups, are most effectively performed whenthe total weight of braze alloy in the mixture does not exceed 40%.

It will be obvious to those of ordinary skill which mixture percentagesshould be applied to which types of structural damage. Accordingly, onepreferred embodiment of the braze alloy mixture of the present inventioncomprises a powder metal slurry. Another preferred embodiment of thepresent mixture invention takes the form of a plasticized powdered metalalloy tape. Another preferred embodiment of this mixture comprises apre-sintered alloy tape. Alternatively, one preferred embodiment of thepresent invention especially useful for dimensional repair comprises apre-sintered alloy preform.

In practice, after the damage has been assessed, the preferred brazealloy composition or compositions of the present invention chosen, thebase metal alloy composition or compositions chosen, and the braze alloyand base metal compositions combined in the appropriate ratiocorresponding to the damage to be repaired, the superalloy article iscleaned of all coatings and oxides using techniques known in the art forsuch cleaning. The chosen braze alloy mixture in the embodimentappropriate to repair the damage, e.g., powder metal slurry, tape, etc.,is then applied to the damaged region and the superalloy articlesubjected to a high temperature brazing cycle in a vacuum or in an inertgas atmosphere. This high temperature brazing cycle melts the brazealloy portion of the mixture, thereby creating a base metal powdermatrix within the braze alloy composition, and joining the entiremixture to the now-repaired superalloy substrate.

One preferred inventive method for repairing damaged superalloycomponents involves a high temperature brazing cycle having thefollowing steps: placing the mixture-coated superalloy article in aninert gas atmosphere or under vacuum in a brazing furnace; obtaining apressure of 1×10⁻³ torr or lower pressure in the inert gas atmosphere orunder the vacuum; heating the braze alloy mixture to a temperature ofabout 800° F. and holding that temperature for approximately 15 minutes;thereafter increasing the temperature to about 1800° F. and holding thattemperature for approximately 15 minutes; then increasing thetemperature again to about 2225° F. and holding that temperature for 15to 45 minutes; whereafter the temperature is vacuum cooled from about2225° F. to about 1800° F.

While the foregoing high temperature braze cycle has been described, itwill be understood by skilled artisans that any series of temperaturesand brazing times capable of melting only the braze alloy compositionand permitting that braze alloy composition sufficient time to flow andeffect the repair while forming a solid solution matrix andprecipitating those particles of the gamma-prime phase are consideredand intended to be encompassed herein. Those of ordinary skill in theart will also understand that the lower the pressure in the brazingfurnace during this brazing cycle, the lower the vapor pressure of thesacrificial oxide forming elements, and thus the better the flow of thebraze alloy during the braze cycle.

The next series of steps in the preferred repair method of the presentinvention comprise the long term diffusion heat treatment cycle. Thisdiffusion cycle is critical to homogenize the remaining solidified brazealloy system microstructure and diffuse the elemental melting pointdepressants into the alloy matrix. The particular steps used in thisdiffusion heat treatment cycle comprise the following: obtaining apressure in the furnace higher than the pressure used in the hightemperature braze cycle, preferably in the range of about 250 torr;heating the mixture deposited on the repaired region to a temperature ofabout 2000° F.; holding the temperature at about 2000° F. forapproximately 2 hours; increasing the temperature to about 2100° F.;holding the temperature at about 2100° F. for approximately 22 hours;and lowering the temperature from about 2100° F. to about 250° F.

While this diffusion cycle may be altered slightly in terms of thetemperatures employed, the range of preferred temperatures for thediffusion braze cycle of the present invention are between 1° and 400°F. less than the highest temperature achieved during the hightemperature brazing cycle. The range of preferred pressures includes anypressure higher than the pressure used in the braze cycle but lower thanatmospheric pressure. Those of ordinary skill will recognize that thehigher the pressure, the less chromium and other elemental vaporizationfrom the repair composite and the superalloy article there will be, andtherefore the less elemental loss there will be.

Additionally, the diffusion braze holding times may vary slightly fromthe holding times described above, but preferred holding times are inthe range of at least 20 hours to about 32 hours in order to permit therepair composite sufficient time to break down the script-like silicidephases into fine discrete particles. Preferred diffusion cycle times arealso adequate both to reduce the size and quantity of brittle boridephases in the repair matrix caused by chromium, titanium, and members ofthe refractory family of elements (tungsten, tantalum, etc.) combiningwith boron, and to diffuse the elemental boron and silicon into therepair composite matrix.

Upon completion of the long term diffusion heat treatment cycle, therepaired part is usually given a new metal or ceramic, diffusion oroverlay coating by means of any known coating method. Such coatingsprotect both the superalloy article and/or the repaired area fromoxidation, hot corrosion, and extreme thermal gradients. Examples oftypical environmental coatings are simple aluminides, platinumaluminides, MCrAl(X)-type overlays, and ceramics. Typical metal coatingssuch as these may be used alone as a single layer coating, as the finallayer of a multilayer coating, or as a bonding coat for a ceramic topcoat; and the ceramic coatings may be used alone directly atop thesuperalloy article surface, or as the final coating atop a bonding coat.However, it is also contemplated by and intended to come within thescope of the present invention to use the present cobalt-base brazealloy compositions as a new type of metal coating which may be used tocoat a superalloy article by means of any coating method. The instantcompositions may also form part of a multilayer coating system in whichthe present compositions are applied to the surface of a superalloyarticle either before or after another environmental coating has beenapplied.

It has been discovered through the course of high temperature cyclicoxidation testing of superalloy parts coated and/or repaired accordingto the present invention that the combination of the present braze alloycomposition(s) with one or more environmental coatings yieldsunexpected, inventive, and beneficial improvements in oxidationresistance. Specifically, the instant cobalt-base braze alloycompositions significantly improve the adhesion of an environmentalcoating to the repair composite. By way of example and not oflimitation, the cyclic oxidation testing was performed at both 2075° F.and 2000° F. on repaired cobalt base metal coupon specimens, and thespecimens of both test conditions exhibited similar results. The testperformed at 2075° F. indicated that the coating spalled off of thecobalt base metal specimens after 40 cycles. The coating did not spalloff the braze repaired areas of the coupons, but it was consumed after300 cycles. The coating around the brazed areas started to spall afterapproximately 100 cycles. At 2000° F., the test results were identical,except the coating over the repair composite lasted over 500 cycles withno loss of coating. It is believed that these surprising achievements inoxidation resistance are a result of the careful balance struck betweenthe oxidation properties and the mechanical properties of the elementsused in the present preferred compositions.

While the invention has been described in detail in the foregoingdescription, the same is to be considered as illustrative and notrestrictive in character, it being understood that only the preferredembodiments have been shown and described, and that all changes andmodifications that come within the spirit of the invention are desiredto be protected.

What is claimed is:
 1. A cobalt-base braze alloy composition, consistingessentially of, by weight:Nickel from about 0.001% to <the weightpercent of cobalt; At least one element selected from the groupconsisting of:Rhenium from about 0.001% to about 15%; Palladium fromabout 0.001% to about 40%; Platinum from about 0.001% to about 40%; Atleast one element selected from the group consisting of:Boron from about0.001% to about 6%; Silicon from about 0.001% to about 10%; and Cobaltbalance.
 2. A cobalt-base braze alloy composition, consistingessentially of, by weight:Nickel from about 0.001% to <the weightpercent of cobalt; One element selected from the group consistingof:Rhenium from about 0.001% to about 15%; Palladium from about 0.001%to about 40%; Platinum from about 0.001% to about 40%; and Cobaltbalance; wherein said braze alloy composition is a eutectic alloy.
 3. Acobalt-base high temperature base metal alloy composition, consistingessentially of, by weight:Nickel from about 0.001% to <the weightpercent of cobalt; Rhenium from about 0.001% to about 15%; At least oneelement selected from the group consisting of:Palladium from about 16%to about 40%; Platinum from about 16% to about 40%; and Cobalt balance.4. The braze alloy composition of claim 1, further comprising, byweight:Aluminum from about 0.001% to about 12%.
 5. The braze alloycomposition of claim 2, further comprising, by weight:Aluminum fromabout 0.001% to about 12%.
 6. The base metal alloy composition of claim3, further comprising, by weight:Aluminum from about 0.001% to about12%.
 7. The braze alloy composition of claim 1, further comprising, byweight:At least one element selected from the group consistingof:Chromium from about 0.001% to about 30%; Tantalum from about 0.001%to about 15%; Molybdenum from about 0.001% to about 15%; Niobium fromabout 0.001% to about 12%; and Tungsten from about 0.001% to about 15%.8. The braze alloy composition of claim 7, further comprising, byweight:At least one element selected from the group consistingof:Titanium from about 0.001% to about 6%; Hafnium from about 0.001% toabout 6%; Iron from about 0.001% to about 3%; Manganese from about0.001% to about 1%; and Zirconium from about 0.001% to about 2%.
 9. Thebraze alloy composition of claim 8, further comprising, by weight:Carbonfrom about 0.001% to about 2%.
 10. The base metal alloy composition ofclaim 6, further comprising, by weight:At least one element selectedfrom the group consisting of:Chromium from about 0.001% to about 30%;Tantalum from about 0.001% to about 15%; Molybdenum from about 0.001% toabout 15%; Niobium from about 0.001% to about 12%, and Tungsten fromabout 0.001% to about 15%.
 11. The base metal alloy composition of claim10, further comprising, by weight:At least one element selected from thegroup consisting of:Titanium from about 0.001% to about 6%; Hafnium fromabout 0.001% to about 6%; Iron from about 0.001% to about 3%; Manganesefrom about 0.001% to about 1%; and Zirconium from about 0.001% to about2%.
 12. The base metal alloy composition of claim 11, furthercomprising, by weight:Carbon from about 0.001% to about 2%.
 13. Thecobalt-base braze alloy composition of claim 9, wherein said compositionconsists essentially of, by weight:Nickel from about 9.5% to about11.5%; Chromium from about 22% to about 24%; Aluminum from about 1.5% toabout 2.5%; Titanium from about 0.75% to about 2.75%; Tungsten fromabout 2% to about 4%; Platinum from 0% to about 40%; Palladium from 0%to about 40%; Rhenium from about 0.001% to about 2%; Tantalum from about5% to about 6%; Carbon from about 0.05% to about 1.05%; Boron from about0.5% to about 2.5%; Silicon from about 4% to about 6%; and Cobaltbalance.
 14. The cobalt-base braze alloy composition of claim 9, whereinsaid composition consists essentially of, by weight:Nickel from about 9%to about 11%; Chromium from about 21.5% to about 23.5%; Tungsten fromabout 6% to about 8%; Rhenium from about 0.001% to about 15%; Tantalumfrom about 2.5% to about 4.5%; Platinum from 0% to about 40%; Palladiumfrom 0% to about 40%; Carbon from about 0.1% to about 1.1%; Boron fromabout 0.5% to about 2.5%; Silicon from about 4% to about 6% Zirconiumfrom about 0.001% to about 1.5% and Cobalt balance.
 15. The cobalt-basebraze alloy composition according to claim 9, wherein said compositionconsists essentially of, by weight:Nickel from about 9.5% to about11.5%; Chromium from about 19.5% to about 21.5%; Aluminum from about1.25% to about 3.25%; Tungsten from about 0.25% to about 2.25%; Rheniumfrom about 0.001% to about 2%; Platinum from 0% to about 40%; Palladiumfrom 0% to about 40%; Hafnium from about 0.001% to about 0.1%; Tantalumfrom about 6.75% to about 8.75%; Carbon from about 0.001% to about0.78%; Boron from about 2% to about 4%; and Cobalt balance.
 16. Thecobalt-base braze alloy composition of claim 9, wherein said compositionconsists essentially of, by weight:Nickel from about 9.5% to about11.5%; Chromium from about 22% to about 24%; Aluminum from about 0.75%to about 2.75%; Tungsten from about 0.25% to about 2.25%; Rhenium fromabout 0.001% to about 2%; Hafnium from about 0.001% to about 1%;Platinum from 0% to about 40%; Palladium from 0% to about 40%; Tantalumfrom about 5.5% to about 7.5%; Carbon from about 0.05% to about 1.05%;Boron from about 1.15% to about 3.15%; Silicon from about 2.25% to about4.25%; and Cobalt balance.
 17. The cobalt-base braze alloy compositionof claim 9, wherein said composition consists essentially of, byweight:Nickel from about 29% to about 31%; Chromium from about 13.75% toabout 15.75%; Aluminum from about 2.3% to about 4.4%; Tungsten fromabout 0.3% to about 2.4%; Rhenium from about 0.001% to about 1.5%;Tantalum from about 7.8% to about 9.8%; Hafnium from about 0.001% toabout 1.5%; Platinum from 0% to about 40%; Palladium from about 2% toabout 4%; Carbon from about 0.001% to about 0.8%; Boron from about 1.3%to about 3.4%; Silicon from about 2.3% to about 4.4%; and Cobaltbalance.
 18. The cobalt-base, base metal alloy composition according toclaim 12, wherein said composition consists essentially of, byweight:Nickel from about 9.0% to about 11.0%; Chromium from about 21.5%to about 23.5%; Aluminum from about 1.0% to about 3.0%; Tungsten fromabout 4.0% to about 6.0%; Rhenium from about 0.001% to about 1.5%;Tantalum from about 5.0% to about 7.0%; Boron from about 1.15% to about3.15%; Platinum from about 16% to about 40%; Palladium from about 16% toabout 40%; Carbon from about 0.05% to about 1.05%; Zirconium from about0.001% to about 1.5%; and Cobalt balance.
 19. The cobalt-base, basemetal alloy composition of claim 12, wherein said composition consistsessentially of, by weight:Nickel from about 9.5% to about 11.5%;Chromium from about 21% to about 23%; Aluminum from about 0.75% to about2.75%; Tungsten from about 3.0% to about 5.0%; Tantalum from about 5.5%to about 7.5%; Rhenium from about 0.001% to about 15%; Platinum fromabout 16% to about 40%; Palladium from about 16% to about 40%; Carbonfrom about 0.05% to about 1.05%; and Cobalt balance.
 20. The cobalt-basebraze alloy composition of claim 8, wherein said composition consistsessentially of, by weight:Nickel from about 9.5% to about 11.5%;Chromium from about 22% to about 24%; Aluminum from about 1.5% to about2.5%; Titanium from about 0.75% to about 2.75%; Tungsten from about 2%to about 4%; Platinum from 0% to about 40%; Palladium from 0% to about40%; Rhenium from about 0.001% to about 2%; Tantalum from about 5% toabout 6%; Boron from about 0.5% to about 2.5%; Silicon from about 4% toabout 6%; and Cobalt balance.
 21. The cobalt-base braze alloycomposition of claim 8, wherein said composition consists essentiallyof, by weight:Nickel from about 9% to about 11%; Chromium from about21.5% to about 23.5%; Tungsten from about 6% to about 8%; Rhenium fromabout 0.001% to about 15%; Tantalum from about 2.5% to about 4.5%;Platinum from 0% to about 40%; Palladium from 0% to about 40%; Boronfrom about 0.5% to about 2.5%; Silicon from about 4% to about 6%;Zirconium from about 0.001% to about 1.5%; and Cobalt balance.
 22. Thecobalt-base braze alloy composition of claim 8, wherein said compositionconsists essentially of, by weight:Nickel from about 9.5% to about11.5%; Chromium from about 19.5% to about 21.5%; Aluminum from about1.25% to about 3.25%; Tungsten from about 0.25% to about 2.25%; Rheniumfrom about 0.001% to about 2%; Platinum from 0% to about 40%; Palladiumfrom 0% to about 40%; Hafnium from about 0.001% to about 0.1%; Tantalumfrom about 6.75% to about 8.75%; Boron from about 2% to about 4%; andCobalt balance.
 23. The cobalt-base braze alloy composition of claim 8,wherein said composition consists essentially of, by weight:Nickel fromabout 9.5% to about 11.5%; Chromium from about 22% to about 24%;Aluminum from about 0.75% to about 2.75%; Tungsten from about 0.25% toabout 2.25%; Rhenium from about 0.001% to about 2%; Hafnium from about0.001% to about 1%; Platinum from 0% to about 40%; Palladium from 0% toabout 40%; Tantalum from about 5.5% to about 7.5%; Boron from about1.15% to about 3.15%; Silicon from about 2.25% to about 4.25%; andCobalt balance.
 24. The cobalt-base braze alloy composition of claim 8,wherein said composition consists essentially of, by weight:Nickel fromabout 29% to about 31%; Chromium from about 13.75% to about 15.75%;Aluminum from about 2.3% to about 4.4%; Tungsten from about 0.3% toabout 2.4%; Rhenium from about 0.001% to about 1.5%; Tantalum from about7.8% to about 9.8%; Hafnium from about 0.001% to about 1.5%; Platinumfrom 0% to about 40%; Palladium from about 2% to about 4%; Boron fromabout 1.3% to about 3.4%; Silicon from about 2.3% to about 4.4%; andCobalt balance.
 25. The cobalt-base, base metal alloy compositionaccording to claim 11, wherein said composition consists essentially of,by weight:Nickel from about 9.0% to about 11.0%; Chromium from about21.5% to about 23.5%; Aluminum from about 1.0% to about 3.0%; Tungstenfrom about 4.0% to about 6.0%; Rhenium from about 0.001% to about 1.5%;Tantalum from about 5.0% to about 7.0%; Platinum from about 16% to about40%; Palladium from about 16% to about 40%; Zirconium from about 0.001%to about 1.5%; and Cobalt balance.
 26. The cobalt-base, base metal alloycomposition of claim 11, wherein said composition consists essentiallyof, by weight:Nickel from about 9.5% to about 11.5%; Chromium from about21% to about 23%; Aluminum from about 0.75% to about 2.75%; Tungstenfrom about 3.0% to about 5.0%; Tantalum from about 5.5% to about 7.5%;Rhenium from about 0.001% to about 15%; Platinum from about 16% to about40%; Palladium from about 16% to about 40%; and Cobalt balance.
 27. Acobalt-base environmental coating composition, consisting essentiallyof, by weight:Nickel from about 0.001% to <the weight percent of cobalt;At least one element selected from the group consisting of:Rhenium fromabout 0.001% to about 15%; Palladium from about 0.001% to about 40%;Platinum from about 0.001% to about 40%; At least one element selectedfrom the group consisting of:Boron from about 0.001% to about 6%;Silicon from about 0.001% to about 10%; and Cobalt balance.
 28. Acobalt-base environmental coating composition, consisting essentiallyof, by weight:Nickel from about 0.001% to <the weight percent of cobalt;One element selected from the group consisting of:Rhenium from about0.001% to about 15%; Palladium from about 0.001% to about 40%; Platinumfrom about 0.001% to about 40%; and Cobalt balance; wherein said coatingcomposition is a eutectic alloy.
 29. The coating composition of claim28, further comprising, by weight:Aluminum from about 0.001% to about12%.
 30. The coating composition of claim 27, further comprising, byweight:At least one element selected from the group consistingof:Chromium from about 0.001% to about 30%; Tantalum from about 0.001%to about 15%; Molybdenum from about 0.001% to about 15%; Niobium fromabout 0.001% to about 12%; and Tungsten from about 0.001% to about 15%.31. The coating composition of claim 30, further comprising, byweight:At least one element selected from the group consistingof:Titanium from about 0.001% to about 6%; Hafnium from about 0.001% toabout 6%; Iron from about 0.001% to about 3%; Manganese from about0.001% to about 1%; and Zirconium from about 0.001% to about 2%.
 32. Thecoating composition of claim 31, further comprising, by weight:Carbonfrom about 0.001% to about 2%.
 33. An environmental coating for asuperalloy substrate comprising:at least one layer of a cobalt-basebraze alloy composition, consisting essentially of, by weight: Nickelfrom about 0.001% to <the weight percent of cobalt; At least one elementselected from the group consisting of:Rhenium from about 0.001% to about15%; Palladium from about 0.001% to about 40%; Platinum from about0.001% to about 40%; At least one element selected from the groupconsisting of:Boron from about 0.001% to about 6%; Silicon from about0.001% to about 10%; and Cobalt balance; wherein said at least one brazealloy layer is positioned over said superalloy substrate; and at leastone layer of a coating material selected from the group consisting of:metal, ceramic; wherein said at least one coating material layer ispositioned over said at least one braze alloy layer.
 34. Theenvironmental coating of claim 33, wherein said metal is selected fromthe group consisting of simple aluminides, platinum aluminides,MCrAl(X)-type compounds, and diffusion braze alloys.
 35. Anenvironmental coating for a superalloy substrate comprising:at least onelayer of a coating material selected from the group consisting of:metal, ceramic; wherein said at least one coating material layer ispositioned over said superalloy substrate; and at least one layer of acobalt-base braze alloy composition consisting essentially of, byweight: Nickel from about 0.001% to <the weight percent of cobalt; Atleast one element selected from the group consisting of:Rhenium fromabout 0.001% to about 15%; Palladium from about 0.001% to about 40%;Platinum from about 0.001% to about 40%; At least one element selectedfrom the group consisting of:Boron from about 0.001% to about 6%;Silicon from about 0.001% to about 10%; and Cobalt balance; wherein saidat least one braze alloy layer is positioned over said at least onecoating material layer.
 36. The environmental coating of claim 35,further comprising at least one additional coating material layerpositioned over said at least one braze alloy layer; whereinsaid atleast one additional coating material layer comprises a coating materialselected from the group consisting of: metal, ceramic.
 37. Theenvironmental coating of claim 35, wherein said metal is selected fromthe group consisting of simple aluminides, platinum aluminides,MCrAl(X)-type compounds, and diffusion braze alloys.